It can be several things
1. You have you geographical location set to ca in webmaster tools
2. You have a .ca domain name which automatically sets your geographical location to ca
3. Your hosting is in Canada, giving you faster speeds there, and slower speeds in USA. Load speed of a webpage is now a ranking factor.
4. All of your links come from sites that have their geographical location set to ca.
These are all factors to consider.

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